1. Collaborators: John Contopoulos Dimitris Christodoulou Denise Gabuzda
Are the AGN Jet Magnetic Fields Chiral? Or Vestiges of a Cosmic Battery D. Kazanas
Collaborators:
John Contopoulos
Dimitris Christodoulou
Denise Gabuzda
SED Director’s Seminar Aug. 14, 2009
Refs: “A Cosmic Battery” Contopoulos, DK (1998)
“The Cosmic Battery Revisited” Contopoulos, DK, Christodoulou (2006)
“Cosmic Battery Simulations in Resistive Accretion Disks” Christodoulou,
Contopoulos, DK. (2008)
“The invariant Twist of Magnetic Fields in the Relativistic Jets of AGN
Contopoulos, Christodoulou, DK, Gabuzda (2009 ApJL 702,148)

2. The Diversity of Cosmic Magnetic Fields
Magnetic fields are ubiquitous in astrophysics over a huge range of scales from stellar compact objects, to stars, to galaxies and clusters of galaxies.
Amogst others, they are the likely driving agent of the jets of Active Galactic Nuclei.

3. Magnetic fields of AGN and clusters of galaxies
The most recent area of study of astrophysical magnetic fields
0.1μG – tens of μG
Huge demands on energy budgets for converting free energy to B-fields.
Organized field at z>2: challenge for dynamo theory

4. The AGN B-Field Geometry
It is expected to be spiral, given that the fields are anchored on the rotating accretion disk.

5. The direction of the spiral is determined by the relative orientation of between the direction of the poloidal field B and the direction of rotation Ω.
Β Ω
Β Ω Vf
Toroidal field BT antiparallel to Vφ
above the disk and parallel below
Toroidal field BT parallel to Vφ above
the disk and antiparallel below

6. CW gradient

7. The direction of the toroidal component can be determined by measurements of the Faraday rotation measure of the plane of linear polarization
B ds is considered positive for B directed toward the observer.
This implies the presence of gradients in χ across the jet that are CW or CCW with respect to the AGN center.
Statistically, there should be equal number of CW and CCW gradients if the poloidal B-field and Ω are random.

8. CW gradient

9. Are the direction of Ω and B independent?
This question reaches to the origin of the cosmic magnetic fields.

10. The magnetic fields of the Sun

11. Large scale magnetic fields in galaxies

12. Interstellar magnetic fields

13. The origin of cosmic magnetic fields is still in doubt.
A homogeneous, isotropic universe cannot support magnetic fields
An inhomogeneous, isotropic universe (scalar inflationary perturbations) also does not support magnetic fields (all vectors are gradients of scalars and their curl is identically zero).
The evolution of B is very similar to that of vorticity w = curl v ( Kulsrud, Cen, Ostriker, Ryu 1997)

14. B-field
vorticity
B-field in expanding
Universe

15. The evolution of B is given by the induction equation

16. The production of magnetic fields requires non-barotropic
i.e. fluids with
Point first made by Biermann (Biermann battery 1950) who
Considered hydrodynamic forces different on electrons and
Protons. This produces very weak fields (10-20 G) which
Presumably get amplified by Dynamo action. This can
amplify the field to equipartition but only on small scales.
We need strong fields organized over large scales.

17. Hydromagnetic dynamo

18. Production of magnetic fields in early universe
Second order perturbation
in photons electrons, coupled
with Compton scattering.
Note the size of the field on
cluster of galaxies scales. Very
hard to bring that up to micro
Gauss with dynamo at the scales
of clusters (not enough time).
Ichiki et al. 2006

19. The simplest way to have
is to employ radiation pressure, strongest near
compact objects.
Implement this in accreting black holes, using the Poynting Robertson effect to impart different velocities to protons and electrons i.e. create a current in the azimuthal direction.

20. The Cosmic Battery p e

21. The Cosmic Battery

22. Criterion: the value of the magnetic Prandtl number
Poynting-Robertson battery
Magnetic field transport
Magnetic field diffusion
Criterion: the value of the magnetic Prandtl number
Pm≡ ν/η ~τmag diffusion / τadvection
Pm>2: saturation
Pm≤2: steady field growth up to equipartition!

23. Aly 1984
Uzdensky & MacFadyen 2006

24. Contopoulos & Kazanas 1998

25. Contopoulos & Kazanas 1998

26. One dimensional model
Contopoulos & Kazanas 1998

27. Two dimensional (semi analytic)
Importance of inner
edge boundary condition
Viscous turbulent disk
joined to ideal MHD wind
with the PR current.
Constant resistivity
throughout the disk.
Eventually magnetic field
diffuses out.

28. Two dimensional (semi analytic)
Pm = 0.001, 0.01,
0.1, 1
Contopoulos, Kazanas & Christodoulou 2006

29. Two dimensional (semi analytic)
With high conductivity interior
to a given radius (e.g. no
turbulence inside LSPO ~6M
Flux trapping interior to that
Radius; field grows to
equipartition.

30. Two dimensional (semi analytic)
Linear growth!
Pm = 0.001, 0.01,
0.1, 1

31. Christodoulou, Contopoulos & Kazanas 2008
Two dimensional numerical z r
Christodoulou, Contopoulos & Kazanas 2008

32. Christodoulou, Contopoulos & Kazanas 2008

33. Christodoulou, Contopoulos & Kazanas 2008

34. Christodoulou, Contopoulos & Kazanas 2008

35. Christodoulou, Contopoulos & Kazanas 2008

36. Christodoulou, Contopoulos & Kazanas 2008

37. Christodoulou, Contopoulos & Kazanas 2008
h = k (DR)2
Christodoulou, Contopoulos & Kazanas 2008

38. Christodoulou, Contopoulos & Kazanas 2008

39. Christodoulou, Contopoulos & Kazanas 2008
The field saturates for low diffusivity (high conductivity)
Christodoulou, Contopoulos & Kazanas 2008

40. Some simple scalings of flux accumulation near x = r/rS = 0
Flux accumulated over
time t (yrs), max~1033 in
a Hubble time
Value of the equipartition
Magnetic field
Time to accumulate field
to equipartition value.
Note the dependence on M

41. The “cosmic” battery
Contopoulos & Kazanas 1998

43. The unique characteristic of the PR Cosmic Battery
In generic MHD disks, the direction of the magnetic field B is decoupled from that of the rotation angular velocity W.
This is not the case with the “Cosmic Battery”: Changing the direction of W changes the direction of the current and guarantees that W and B are always parallel!! (in the interior field and antiparallel in exterior)
This has direct consequences for the toroidal component of the magnetic field BT

44. The invariance of the toroidal B-field direction
B, W _ + Vf Vf
B, W

45. RM = (constants)  B•dl
 = o + RM 2
The direction of the poloidal field can be obtained through Faraday Rotation Measure observation which determine the value and sign of  B•dl. One can see that the gradient of RM is invariant and it is in the CW direction with respect to the AGN center for the inner field and in the CCW direction for the return field.

46. VLBI observations of 3C 78 and 1749+701

48. Field geometry suggested by Mahmud & Gabuzda for 1803+784

49. CW gradient, CCW gradient, gradient of largest scale in 2 scale measurements
We have compiled a list of 36 observations of reliable RM gradients, with 22 CW and 14 CCW directions (chance probablility 12%)
However, if we look at the gradients closest to the core we have 22 CW and only 7 CCW gradients with chance probability 0.6%.
Also if we look at the gradients at largest distances from the core we find that all 7 are in the CCW direction with similar chance probability ~0.8%.

50. At kpc scales (VLA) jets are disturbed in interactions with the ambient medium and it is hard to discern clear-cut cases. We (CGCK 15, submitted) were able to find 7 good cases, all in the CCW direction.

51. Conclusions
The PR battery appears to be the most efficient agent for large scale magnetic field generation.
It provides a secular source for magnetic fields, most efficient in compact objects powered by accretion where equipartition fields can be reached in short time.
The toroidal component of PR battery fields are chiral, i.e. their twist has a unique sense independent of the rotation direction of the disk. This feature is unique to the specific mechanism and observations provide a strong support of this notion.
AGN besides the main sources of high energy radiation (X-, γ-rays) in the Universe appear to be also a (the?) major source of magnetic flux, i.e. cosmic magnetic fields.